Double metal cyanide complex catalysts were discovered in the decade of the 1960s, and were found to have significant catalytic activity in a variety of reactions, particularly polymerizations. Although double metal cyanide salts themselves were found to have little or no catalytic activity, non-stoichiometric complexes formed from the double metal cyanide salt and an organic complexing agent were found to possess high activity. The activity was found to vary with the identity of the metals contained in the complex, and also with the organic complexing agents. The chemical makeup, effects of varying metal ions, and differences in reactivity due to the complexing agent are discussed in U.S. Pat. No. 3,427,335, herein incorporated by reference, which further indicates that polymers of different intrinsic viscosities, and therefore of differing molecular weight, may be obtained by suitable selection of organic complexing agent.
According to the '335 disclosure, excess complexing agents can be removed by extraction with a low boiling, non-complexing solvent such as pentane or hexane. In a typical laboratory catalyst preparation, a solution of an alkali metal hexacyanometallate salt, e.g. K.sub.3 Fe(CN).sub.6 is added slowly to a stirred solution of metal chloride salt, e.g. zinc chloride, in slight molar excess. The precipitated zinc hexacyanoferrate(III) salt is washed thoroughly with water, and then washed with three portions of anhydrous dioxane. In an optional procedure, the dioxane washed precipitate is slurried in dioxane/hexane and refluxed, water being removed as an azeotrope. The moist solid is dried under vacuum of c.a. 1 torr. The dry catalyst may be crushed to a fine powder.
The catalytic activity of catalysts of the type disclosed by the '335 patent and other related disclosures such as U.S. Pat. Nos. 3,427,256, 3,427,334, 3,829,505, and 3,941,849, although high, was not high enough to overcome the high cost of such catalysts relative to other catalysts traditionally utilized. For example, in conventional oxyalkylation reactions useful in preparing polyoxyalkylene polyols and polyoxyalkylene block surfactants, potassium hydroxide had long been the catalyst of choice due to its low cost. Moreover, removal of catalyst residues from double metal cyanide catalyzed polyols also proved to be problematic and to add additional expense to the production process. As a result, little if any commercialization of double metal cyanide catalysts of the types disclosed by the aforementioned patents occurred.
In the 1980's, double metal catalysts were revisited, spurred on in part by the desire to manufacture polyether polyols with lower unsaturation and higher equivalent weights. In base catalyzed polyoxyalkylation, a competing rearrangement of higher alkylene oxides into unsaturated alcohols continuously introduces monofunctional, oxyalkylatable species into the oxyalkylation reactor. For example, propylene oxide, the most widely used higher alkylene oxide, rearranges to allyl alcohol. Oxypropylation of this monohydric species results in polyoxyalkylene monols. Continued generation of allyl alcohol and the continued oxyalkylation of it and the previously generated and oxyalkylated monols results in a considerable proportion of monohydric species spanning a broad molecular weight range.
For example, in the manufacture of polypropylene glycols, the base catalyzed oxypropylation of a propylene glycol initiator results in a mixture of polyoxypropylene glycols and oxypropylated allyl alcohol polymers and oligomers. As oxypropylation continues, the mol percentage of monofunctional species steadily increases. In a 2000 Da equivalent weight polyoxypropylene "diol," the monofunctional species content may range between 30 and 40 mol percent, and the functionality reduced from the "nominal," or theoretical functionality of 2.0 to an actual, measured functionality in the range of 1.6 to 1.7. In the case of a 2000 Da equivalent weight triol, e.g. an oxypropylated glycerine polyol, the actual functionality will be closer to two than the nominal, or "theoretical" functionality of three.
Investigations of other catalysts in attempting to lower monol production during oxyalkylation did not, in general, lead to commercially acceptable systems. For example, lowering the reaction temperature during base catalyzed oxypropylation was found to lower unsaturation, but at the expense of greatly increased process time. Levels of unsaturation in the range of 0.010 meq/g polyol, as measured by ASTM 2849-69, "Testing of Urethane Foam Polyol Raw Materials" could be produced, but with reaction times measured in days or even weeks rather than typical batch times of 8 to 12 hours. Use of alternative catalysts such as cesium or rubidium hydroxide (U.S. Pat. No. 3,393,243); strontium or barium oxides and/or hydroxides (U.S. Pat. Nos. 5,010,187 and 5,114,619); and alkaline earth metal carboxylates (U.S. Pat. No. 4,282,387) have all been proposed.
In U.S. Pat. Nos. 4,472,560 and 4,477,589, promoted double metal cyanide complex catalysts prepared by addition of inorganic acids or salts such as alkali metal hexafluorosilicates to double metal cyanide complexes were proposed. The promoter addition takes place in the presence of excess complexing agent, i.e. glyme, or in the presence of a liquid initiator, and following dehydration produces a catalyst/initiator slurry. However, a different slurry must be prepared for each different initiator desired, and the process cannot be used to prepare slurries of catalyst in volatile initiators. Moreover, the catalyst slurries are much more expensive to ship as compared to dry catalyst. However, the catalysts were stated to exhibit improved catalytic activity, and were also stated to be useful at temperatures in the range of c.a. 110.degree. C. to 120.degree. C., while prior DMC catalysts generally were rapidly deactivated at temperatures in excess of 100.degree. C.
Further improvements in DMC catalysts are evidenced by the processes of preparation disclosed in U.S. Pat. No. 5,158,922, wherein modestly heated double metal cyanide-forming reactants, a relatively large stoichiometric excess of metal salt over metal cyanide salt, and a specific order of mixing these salts resulted in greatly improved catalytic activity. Japanese Patent Application Kokai No. 4-145123 disclosed that use of t-butanol as the organic complexing agent rather than glyme, the most common complexing agent, also resulted in improved catalysts, particularly with respect to catalyst longevity. These improvements, coupled with improved and less costly methods of removal of catalyst residues from finished polyether products as illustrated by U.S. Pat. Nos. 4,721,818; 4,987,271; 5,010,047; and 5,248,833, led to commercialization of DMC-catalyzed polyether polyols for a short time.
Most recently, discoveries by the ARCO Chemical Co. have resulted in double metal cyanide complex catalysts which not only offer polymerization rates which are considerably higher than prior catalysts, but moreover are far more easily removed from the polyoxyalkylene polyether product. While earlier DMC catalysts were able to produce polyols with levels of unsaturation in the range of 0.015-0.020 meq/g, these new catalysts consistently produce polyols with unsaturation in the range of 0.003 to 0.008 meq/g. Such catalysts are disclosed in U.S. Pat. Nos. 5,470,813 and 5,482,908, which are incorporated herein by reference. Double metal catalysts such as those disclosed by the 5,470,813 and 5,482,908 patents often allow for catalyst residue removal from polyol product by simple filtration. Moreover, the catalytic activity is so high in some cases that the low amounts of catalyst used, e.g. 10-25 ppm, does not require any removal process.
However, the process of preparing the double metal cyanide complex catalysts themselves is lengthy, and involves numerous steps. While the process is easily done on a laboratory scale, on a commercial scale, catalyst preparation time increases dramatically. For example, in a commercial scale manufacturing process, catalyst preparation may consume in excess of 100 hours. Approximately 88% of this time is consumed in isolating the catalyst solids, drying the moist filter cake obtained, and grinding the catalyst into small particles.
Surface morphology may also be of importance with respect to catalytic activity for double metal cyanide complex catalysts. For example, in U.S. Pat. No. 5,470,813, unique double metal cyanide catalysts were produced which differed from prior art catalysts by being substantially amorphous, rather than possessing significant amounts of highly ordered or crystalline material. The amorphous nature of these catalysts was demonstrated by the lack of certain sharp lines in the X-ray diffraction spectrum which are characteristic of crystalline double metal cyanide salts.
The substantially amorphous catalysts exhibited surprising and unexpected increases in catalytic activity, yet the particle size was actually much larger than that of prior art catalysts, prepared from similar chemical constituents, which thus presented higher surface area. The catalytic activity of such substantially amorphous catalysts can be increased yet further by grinding the catalyst to smaller particle sizes. Particle sizes less than 10 .mu.m are desired.
The grinding process is very time intensive. Moreover, the moist, bulk filter cake produced during catalyst preparation retains a substantial amount of complexing agent, even after considerable time drying in vacuo. During this intensive grinding, surface modifications to the catalyst particles due to the inherent nature of the grinding operation may cause changes in catalytic activity. Thus, an increase in activity due to smaller particle size may be offset, at least in part, by a decrease in activity due to changes in surface morphology. Surface morphology may also affect properties other than activity per se. For example, double metal cyanide catalysts produced in finely ground form may also exhibit reactor fouling, in which gel-like and presumably very high molecular weight products accumulate in the reactor.
It would be desirable to provide a process by which double metal cyanide complex catalysts may be prepared with reduced processing time. It would be further desirable to be able to prepare double metal cyanide complex catalysts of small particle size without the risk of altering surface morphology by intensive grinding. It would be yet further desirable to prepare double metal cyanide complex catalysts which offer increased handling ease, increased storage stability, less reactor fouling during polymerization, and higher catalytic activity.